Re: node-hotplug: is memset 0 safe in try_offline_node()?

[Xishi Qiu <qiuxishi@huawei.com>]

On 2015/3/4 11:56, Gu Zheng wrote:

> Hi Xishi,
> On 03/04/2015 10:52 AM, Xishi Qiu wrote:
> 
>> On 2015/3/4 10:22, Xishi Qiu wrote:
>>
>>> On 2015/3/3 18:20, Gu Zheng wrote:
>>>
>>>> Hi Xishi,
>>>> On 03/03/2015 11:30 AM, Xishi Qiu wrote:
>>>>
>>>>> When hot-remove a numa node, we will clear pgdat,
>>>>> but is memset 0 safe in try_offline_node()?
>>>>
>>>> It is not safe here. In fact, this is a temporary solution here.
>>>> As you know, pgdat is accessed lock-less now, so protection
>>>> mechanism (RCUi 1/4 ?) is needed to make it completely safe here,
>>>> but it seems a bit over-kill.
>>>>
>>
>> Hi Gu,
>>
>> Can we just remove "memset(pgdat, 0, sizeof(*pgdat));" ?
>> I find this will be fine in the stress test except the warning 
>> when hot-add memory.
> 
> As you see, it will trigger the warning in free_area_init_node().
> Could you try the following patch? It will reset the pgdat before reuse it.
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 1778628..0717649 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1092,6 +1092,9 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
>                         return NULL;
>  
>                 arch_refresh_nodedata(nid, pgdat);
> +       } else {
> +               /* Reset the pgdat to reuse */
> +               memset(pgdat, 0, sizeof(*pgdat));
>         }

Hi Gu,

If schedule last a long time, next_zone may be still access the pgdat here,
so it is not safe enough, right?

Thanks
Xishi Qiu

>  
>         /* we can use NODE_DATA(nid) from here */
> @@ -2021,15 +2024,6 @@ void try_offline_node(int nid)
>  
>         /* notify that the node is down */
>         call_node_notify(NODE_DOWN, (void *)(long)nid);
> -
> -       /*
> -        * Since there is no way to guarentee the address of pgdat/zone is not
> -        * on stack of any kernel threads or used by other kernel objects
> -        * without reference counting or other symchronizing method, do not
> -        * reset node_data and free pgdat here. Just reset it to 0 and reuse
> -        * the memory when the node is online again.
> -        */
> -       memset(pgdat, 0, sizeof(*pgdat));
>  }
>  EXPORT_SYMBOL(try_offline_node);
>  
> 
>>
>> Thanks,
>> Xishi Qiu
>>
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> 
> 
> 
> .
> 



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